Chapter 15
Crustal Deformation
Notes
(2 classes notes + 1 class Xword & review)
Objectives:
1. Define crustal deformation, force, and stress. p. 415
2. Describe elastic, brittle, and ductile deformation. pp. 416-418
3. Describe the factors that affect deformation. p. 415-419
4. Define faulting. p. 425
5. Distinguish between dip-slip, strike slip, and transform faults.
pp. 427-432
6. Define folding. p. 421
7. Relate compressional, tensional, and shear forces to various
types of crustal deformations such as faults and folds, p. 415
8. Relate faulting and folding to mountain building. pp. 427-429
Crustal Deformation – refers to all changes in the original form
and/or size, and/or location and/or orientation of a rock body.
Most crustal deformation occurs along plate boundaries but it can
also occur at other places like hotspots. It includes changes in the
rock body such as folds, fault, uplifting, subsidence, and
overturning.
Crustal deformation falls into three (3) categories:
i. Elastic – when stress is applied to a rock body, the rock will
first deform elastically. This means that if the stress is
removed, the rock will return to nearly its original size and
shape. Elastic deformation is associated with earthquakes.
ii. Ductile – once the rock has reached the limit to which it can
be deformed elastically, it will either flow (stretch or shrink)
or break (fracture). Both of these deformations are
permanent. If the rock flows but doesn’t break, the
deformation is said to be ductile.
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iii. Brittle – if the rock breaks, the deformation is said to be
brittle.
Force – a push or pull which tends to make stationary objects
move or change the motion of bodies already moving.
Stress – the amount of force to a given area. It can be uniform in
all directions (uniform stress) or concentrated in certain directions
(differential stress).
The forces/stresses that act on rock bodies tend to be of three
types (see pic below) :
i. Compressional – this force tends to shorten a rock body
(see pic A).
ii. Tensional – this force tends to lengthen a rock body (see pic
B).
iii. Shear – cause rock bodies to slide past each other (see pic
C). See animation here.
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Factors Affecting Deformation
1. Temperature – The colder the rock, the greater the chance
that it will break when deformed (brittle). The warmer the
rock, the greater chance that the deformation will be ductile
or elastic.
2. Confining pressure – this will only result in elastic or
ductile deformation. Brittle deformation usually occurs on
the surface where there is no pressure from above.
3. Rock type – All rocks can be deformed, but sedimentary
rocks are usually easier to deform since they are softer.
4. Time – Rapid deformation tends to result in brittle
deformation (fracturing). Slow deformation usually results in
ductile or elastic deformation.
Faulting –the breaking of rock layers and their subsequent
movement. (break and move). Faulting usually occurs when
rapid, continual forces, acting near the surface, cause brittle
deformation.
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Types of Faults
1. Dip-slip faults - these are faults in which the movement is
primarily vertical. Since the movement can be vertically up or
vertically down, there are two types of dip-slip faults. To
distinguish between them we call the rock above the fault the
hanging wall and the rock below the fault the footwall (see Figure
15.20 p. 427 text). See pic here
i.
Normal Faults - occur when the hanging wall moves
downward relative to the foot wall (see Figure 15.21 p. 428
text). The force is tensional. (See p 415 & Figure 15.2 C p.
416 text). See animations here, here, here & here Normal
faults are common at divergent boundaries. At this boundary,
a normal fault can occur on each side of a large block of crust
which drops as the plates move apart. This block is known as
a graben (see Figure 15.23 p. 429 text). See animations here
& here
ii. Reverse Faults - occur when the hanging wall moves upward
relative to the foot wall (See Figure 15.24 p. 430 text). If the
reverse fault has a very low angle, the fault is called a thrust
fault (see Figure 15.25 p. 430 text) and here. The force is
compressional. (See p. 415 & Figure 15.2 B p. 416 text). See
animations here, here, here & here
Reverse faults are common at convergent boundaries. At this
boundary, a reverse fault can occur on each side of a large
block of crust which rises as the plates move together. This
block is known as a horst (see Figure 15.23 p. 429 text). See
animations here & here
2. Strike-slip Fault - these are faults in which the movement is
primarily horizontal (see Figure 15.27 & 15.28 p. 431 text). Large
strike-slip faults associated with plate boundaries are called
transform faults. The force is a shear force (see p. 415 & Figure
15.2 D p. 416 text). See animations here, here, here, and here. A
famous example is the San Andreas fault in California which is the
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main cause of the earthquakes in that state. See offset here and
here, and here, and all three faults here.
Sample Exam Questions
1. What type of fault would result from the forces indicated?
(A) normal
(B) reverse
(C) strike-slip
(D) thrust
2. What type of stress causes reverse faulting?
(A) compressional
(B) expansion
(C) shearing
(D) tension
Folding – the bending of rock layers. Folding results from the
slow, continual, compressional forces acting fairly deep in the
crust. See animations here.
Types of Folds
There are two common types of folds that often occur near each
other. See animation here
1. Anticline – formed by upfolding, or arching, of rock layers.
Think of the crest of a wave. (see Figure 15.10 p. 422 text) See
pic here. Anticlines are sometimes the location of crude oil
deposits. If you draw a horizontal line through the anticline,
you will make a rounded “A” shape. (A for Anticline)
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2. Syncline – formed by downfolding of rock layers. Think of the
trough of a wave. (see Figure 15.10 p. 422 text) See pic here.
Synclines are sometimes the location of water deposits. Signal
Hill in St. John’s is located on a syncline.
Mountain Types, Faulting & Folding
There are 4 basic types of mountains and each has faulting and
folding associated with it. We will examine three of those types
briefly. Volcanic mountains are done in detail in chapter 4.
1. Fault-Block Mountains - these mountains form as a result of
tensional stresses and they contain normal faults with a high to
moderate angle (see Figure 15.22 p. 429 text). These mountains
typically consist of several nearly parallel mountain ranges. See
here (6). The Sierra Nevada Mountains in California are an
example.
2. Folded Mountains - these are the largest and most complex
mountains. They form as a result of compression forces. Folded
strata are most evident in these mountains although large thrust
fault are also present. Examples are the Urals and Himalayas. See
video here
3. Upwarped (Domed) Mountains - these are formed when the crust
arches upward or when one side of a fault moves a great distance
vertically because of a force underneath the crust. Folding will
occur where the crust arches. Examples are the Black Hills in
South Dakota and the Adirondacks.
Do #'s 4, 14, 15, 17, 18, 19 p. 437 text.
Read Chapter Summary items 1, 5-7 p. 436 text.
Read pp. 441-455 for next day.
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